dap1--- via Mailman-users writes:
I don't have a static IP and don't want to pay for one or change my ISP. I've had my email address for far too many years to change it now.
Are you saying your Mailman list-post mailbox is your personal mailbox, perhaps using an mailbox+listname extended address to distinguish list traffic from personal mail? And you're distributing via Mailman to third parties as well as back to yourself?
And while you're answering questions, how about showing us the From header?
Really, you need to stop trying to diagnose the problem yourself ("must be Mailman, Postfix, or fetchmail") -- we're just going to ignore that -- and start giving us some facts to work with.
-- GNU Mailman consultant (installation, migration, customization) Sirius Open Source https://www.siriusopensource.com/ Software systems consulting in Europe, North America, and Japan